The present invention pertains to the field of fabricating bulk acoustic wave resonator, and more particularly to the field of fabricating, on a single substrate, more than one filter, each filter including at least two such resonators.
It is known in the art to provide a so-called bulk acoustic wave (BAW) resonator, which includes a piezolayer sandwiched between two metallic layers that serve as electrodes. Such an assembly has a resonant frequency, with the thickness of the piezolayer the predominant factor in determining the resonant frequency, and so is often used as a component of a radiofrequency (RF) filter, as in for example mobile phone equipment. A typical example of such a filter is the so-called ladder filter, which often includes two such resonators, a series resonator (placed in series with the signal to be filtered) and a shunt resonator (shunting the signal to be filtered), and in fact can include a series combination of several such resonator pairs. (Other known filter types exist as well, such as a so-called lattice filter.) Besides the thickness of the piezolayer, the thickness and material used for others of the layers of a BAW resonator affect the resonant frequency of the resonator. In case of a series and shunt resonator pair, the two resonators are usually made the same except for adding a thin tuning layer to one or the other to shift slightly its resonant frequency compared to the other of the pair. The pair, which can serve by itself as a filter or as one stage in a ladder filter of several stages of such pairs, thus has what is called a center frequency, i.e. a frequency essentially midway between passband edges and so close to the series resonance frequency of the series resonator and at or close to the parallel resonant frequency of the shunt resonator.
BAW resonators according to the prior art are either bridge-type resonators, fabricated on one face of a thin membrane, with an air interface below the membrane and another air interface on top of the upper surface of the resonator, or acoustic-mirror type resonators fabricated on one face of a so-called acoustic mirror, which is in turn deposited on a substrate. An acoustic mirror consists of layers of various materials having alternately high and low acoustic impedance each having an approximately xcex/4 thickness.
Both acoustic mirrors and membranes are here called isolation structures, because both structures acoustically isolates a BAW resonator section, consisting of the piezolayer and electrodes, from the underlying substrate.
In fabricating a BAW resonator, a process is used in which many BAW resonators are fabricated on what is termed a wafer, a usually four to eight-inch diameter silicon or glass disk. Usually thousands of BAW resonators are fabricated on such a wafer and then the wafer is sawed into individual chips. The portion of the wafer included in a chip is called here a substrate, to distinguish it from the whole wafer.
With the advent of multiband mobile phones, there is motivation to provide greater integration of those multiband mobile phone components that provide similar functionality, such as all components that provide filtering. Rather than providing a discrete RF filter for each frequency received or transmitted by a multiband mobile phone, it would be advantageous to provide the different filters on a single substrate (which would be sawed from a wafer on which thousands of such filters would be fabricated). However, although it is known in the art to provide both BAW resonators of a single-stage filter on a single substrate, or several such pairs making up a ladder filter on a single substrate, the prior art does not teach how to overcome the obstacle of fabricating on a single substrate several BAW filters with substantially different center frequencies and so having substantially different thicknesses of the piezolayer. Instead, the prior art teaches fabricating different filters (with substantially different center frequencies) on different substrates, and then combining the individually packaged filters into one module, a module that is unavoidably larger and usually more costly than a single-substrate multi-band filter.
Therefore, what is needed is a method of fabricating several BAW filters with substantially different center frequencies on a single substrate, a method that ideally includes only a small number of additional steps compared to the process of providing a single BAW filter on a substrate.
Accordingly, the present invention provides a method of fabricating a plurality of bulk acoustic wave (BAW) resonators on a single substrate, and the corresponding product, the BAW resonators having substantially different resonant frequencies, the method including the steps of: providing a substrate having an upper facing surface; depositing an isolation structure on the upper facing surface of the substrate; depositing a first metallic layer on the isolation structure, the first metallic layer serving as a bottom electrode; and depositing piezolayer material on the bottom electrode so as to have thicknesses corresponding to each of the different resonant frequencies, each different thickness located in a location where a resonator having a resonant frequency corresponding to the thickness is to be located.
In a further aspect of the invention, the step of depositing piezolayer material on the bottom electrode itself includes the steps of: depositing piezolayer material to a thickness corresponding to the lowest frequency resonator; providing hard mask material over areas where the lowest frequency resonators are to be located; and removing the piezolayer material down to the thickness of the next higher frequency resonators.
In another, further aspect of the invention, the step of depositing piezolayer material on the bottom electrode itself includes the steps of: depositing the piezolayer material to a thickness corresponding to the highest frequency; depositing a lift-off mask where the highest frequency resonators are to be located; depositing additional piezolayer material to a thickness corresponding to the next highest frequency; and removing the lift-off mask.
In yet still another, further aspect of the invention, the step of depositing piezolayer material on the bottom electrode itself includes the steps of: depositing the piezolayer material to a thickness corresponding to the highest frequency; depositing hard mask material where the highest frequency resonators are to be located; depositing additional piezolayer material to a thickness corresponding to the next highest frequency; and depositing hard mask material where the next highest frequency resonators are to be located.
In yet still even another, further aspect of the invention, the isolation structure is an acoustic mirror, and the method further includes the step of providing the acoustic mirror, interposed between the substrate and the bottom electrode, according to a design that imparts to the acoustic mirror a desired reflection coefficient over a predetermined range of frequency including the substantially different resonant frequencies.